A few new DS demos are now on the Nintendo Channel, though sadly the Ninja Gaiden demo is gone. New ones are:
Brain Age, The Spiderwick Chronicles, Apollo Justice: Ace Attorney, Eco-Creatures: Save the Forest, My Word Coach, and Iron Man.
All but Brain Age have an expiration date on 5/18/08, so you have a week to play them. I wish we knew that forst last week's title so that i could've gotten more playtime with Ninja Gaiden.
Also worth noting, Brain Age 2, Crosswords DS, and Flash Focus are all still available for download. It seems like Nintendo's own demos wont expire, hence why Brain Age has no expiration, where as this week's 3rd party titles do.